Conversely: although SNPs in the MAPT gene do not influence the risk of having Alzheimer's, they appear to lead to increased MAPT mRNA production, which leads to increased levels of the tau protein in cerebrospinal fluid, which ultimately led to an earlier age of onset of Alzheimer symptoms (in those who eventually developed Alzheimer's). So if - and only if - a person is fated to develop Alzheimer's for other reasons, then these MAPT SNPs are correlated with exhibiting symptoms sooner. PMID 18541914

One hypothesis is that genes relevant to viral infection, and in particular herpes simplex virus (HSV-1) infection, may increase the risk for Alzheimer's, especially in individuals already predisposed to Alzheimer's, such as those carrying APOE-ε4 alleles. PMID 16406033 In connection to this hypothesis, rs2254958, a SNP in a gene influencing HSV-1 infection, has been found in higher frequency in certain AD patients. PMID 17420072

SNPs defining a haplotype of the TRPC4AP gene on chromosome 20q11.22 appear to be associated with late-onset Alzheimer's disease in two large affected families. The relevance outside of these families is unclear. PMID 18449908